1. Technical Field
The present invention relates to a projector employing a liquid crystal cell as an optical means of modulation (liquid crystal projector). The present invention relates particularly to so-called liquid crystal light-valve that employs only a single liquid crystal light-valve and performs color display (single-light-valve type liquid crystal projector).
2. Prior Art
The cathode-ray-tube (CRT) method has been employed for many years as an apparatus which projects and displays on an enlarged scale an image, such as a television. In contrast with this, there is a method that displays such an image by the use of a liquid crystal projector (liquid projector method). The liquid crystal projector method, as compared with the CRT method, has excellent characteristics such that a range of color reproduction is wide, miniaturization and lightening of weight are easy, and adjustment of convergence is not needed.
On one hand, the liquid crystal method, as compared with the CRT method, has the commercial disadvantage that the cost is high, in addition to the disadvantages of the display characteristics that the luminance is small and the dynamic-image display characteristic is not good. However, it is also undeniable that the liquid crystal method has the advantages of the display characteristics that uniform luminance is obtained on a screen and there is less flickering of a screen. In the comparison of both methods such as this, what is regarded as a serious problem is the cost of the liquid crystal method. The problem of cost is related to an enhancement in the luminance of the liquid crystal projector. That is, since a liquid crystal device itself does not emit light, there is a need to provide an additional light source and accordingly a high-luminance light source takes an additional cost. In addition, since the size is increased, the advantages of the liquid crystal method cannot be utilized.
In addition to this, the cost of the liquid crystal device itself has become a problem. When performing color display with the liquid projector method there is a method which uses three liquid crystal panels (three-panel method). This method is a method which transmits red (R) light, green (G) light, and blue (B) light to three respective liquid crystal panels, optically superposes the R, G, and B lights modulated by these liquid crystal panels, and obtains a single full color image. However, this method uses three liquid crystal panels, and consequently it gives rise to a serious problem of costs and may sacrifice miniaturization and lightening of weight which are the original advantages of the liquid crystal projector method.
Hence, a method of performing color display by the use of a single liquid crystal panel alone (single-panel method) has become the main trend in recent years. The representation of the single-panel type liquid crystal display unit or the liquid crystal projector, as is disclosed in Published Unexamined Patent Application No. 59-230383, has the structure where white light is irradiated by an optical system to a liquid crystal display panel equipped with a three-primary color filter pattern in mosaic or a stripe form. However, in this method, since light is all transmitted through the color filter, only about one-third of light emitted from a source of white light is utilized. Therefore, in order to obtain similar luminance as the three-panel method with the single-panel method, there is a need to use a light source bright by three times or more, however, there arises the problem that the increases in the size and weight and the high cost of an apparatus are caused. Such a reduction in the efficiency of light utilization is fatal to the liquid crystal projector method not only in the aspect of luminance but also in the aspect of costs.
A single-panel type liquid crystal projector method which has solved the aforementioned problem of luminance (efficiency of light utilization) is disclosed in Published-Unexamined Patent Application No. 4-60538. This method succeeds in removal of a color filter and makes absorption of light by a color filter zero. This method is one which irradiates white light to a plurality of dichroic mirrors arranged at angles slightly different from a direction in which light travels, divides the irradiated light into R, G, and B light beams, and transmits these R, G, and B light beams to the incident surface of a liquid crystal panel at different positions for each light beam through a micro lens array (new single-panel method). The present invention is one which has been based on a liquid projector according to the new single-panel method and has improved this.
The liquid crystal projector of the new single-panel method will be described in detail in reference to FIG. 3. White light is emitted from a light source 1 and made into a nearly collimated beam of light by a condenser lens 3. Thereafter, the white light is changed in direction by three dichroic mirrors 5 slightly different in angle and is reflected in a direction in which a liquid crystal panel 9 is arranged. The dichroic mirrors 5 have a function of separating white light into R, G, and B lights and also reflecting the separated R, G, and B lights in three directions slightly different from each other. For example, the dichroic mirror 5.sub.R related to R light is arranged at an angle of .theta.=44.5.degree., the dichroic mirror 5.sub.G related to G light is arranged at an angle of .theta.=45.degree., and the dichroic mirror 5.sub.B related to B light is arranged at an angle of .theta.=45.5.degree. in the figure. The R, G, and B lights separated by these dichroic mirrors 5 are incident at angles (e.g., -1.degree., 0.degree., and 1.degree.) slightly different to the normal line of the incident surface of the liquid crystal panel 9 through a lens array 7.
To show this detail, the enlarged view of a dotted line portion 20 in FIG. 3 is shown in FIG. 4. White light is separated into light beams with a plurality of colors which are incident upon the liquid crystal panel 9, and the separated light beams are transmitted through the lens array 7 having convex surfaces at pitches nearly three times the cell pitch of the liquid crystal panel 9. The focal point of the lens array 7 is on the plurality of incident surfaces of the liquid crystal panel 9. Therefore, the light beams are focused on the incident surface of the liquid crystal panel 9 and produce fringes in order of B, G, and R. If a single pixel of the liquid crystal panel is formed in correspondence with the position of the fringes, it will be possible to perform color display with a single liquid crystal panel by controlling transmittance for each pixel.
Returning to FIG. 3, the light beams modulated for each pixel in the aforementioned way by the liquid crystal panel 9 are transmitted through the liquid crystal panel with controlled transmittance. After the transmitted light beams have been converged by a field lens 11, they are magnified by a projection lens 13 and are projected on a screen 15.
Such a new single-panel method makes an epoch in that high luminance can be realized by removing a color filter when performing color display. However, as with the single-panel method, a single color pixel is constituted by three pixels of a liquid crystal panel, so there arises the problem that the resolution of the liquid crystal panel itself needs to be enhanced by three times compared with the three-panel method. For example, the new single-panel method requires a liquid crystal panel with 92.times.10.sup.4 cells for a VGA color display of 640 by 480 dots, however, a liquid crystal panel such as this becomes expensive, and on top of that, the aperture efficiency become reduced and the projection screen becomes dark because the rate at which thin-film transistors (TFT) and wires occupy becomes high. If this problem is attempted to be solved, the size of the panel will be increased. Consequently, an increase in the size of peripheral parts such as a lens and a mirror will lose the original advantages of the liquid crystal projector such as light weight and small size.
In order to eliminate the aforementioned disadvantages of the new single-panel method, structure employing high-speed response liquid crystal material such as ferroelectric liquid crystal material developed in recent years, such as that shown in FIG. 5, is conceivable as a method of realizing color display with a single liquid crystal panel without enhancing resolution. This method is a method which separates colors from each other not by employing a plurality of dichroic mirrors but by transmitting white light through a rotating color filter 17 and uses a single pixel of a liquid crystal panel in sequence for each of R, G, and B colors (color sequencing method). This method does not have the problem of resolution as does the new single-panel method, because a single color pixel is produced with a single pixel of a liquid crystal panel by making use of an illusion of an image remaining on an eye. The other parts in FIG. 5 basically have a similar function as those described in FIG. 3 and therefore the same reference numerals are applied to the other parts.
According to the color sequential method, resolution is reliably enhanced compared with the new single-panel method and also there is a less problem of an increase in the size and a rise in the cost of an apparatus. However, since a color filter is employed for separation of colors, there arises the problem that luminance which is the disadvantage of the old single-panel method is low.
The current known methods for a liquid crystal projector, as previously described, have a problem of either resolution or luminance, and there is not known method meeting all of these. The problem that is solved by the present invention is to devise a liquid crystal projector method in which resolution and luminance are favorable, by taking the advantages of the color sequential method ori the basis of the new single-panel method and combining these both methods together.